The present invention relates to a method of making a ribbon type coated optical fiber.
The making of a ribbon type coated optical fiber comprises the steps of arranging a plurality of already manufactured optical fibers (referring to glass fibers having their outer periphery coated with a resin) in alignment with each other; passing the optical fibers through a die to which a coating resin is supplied, so as to collectively coat the optical fibers with the resin into a ribbon form; curing the resin with a curing apparatus; and taking up the resulting ribbon type coated optical fiber with a take-up apparatus by way of a capstan, a pulley, and the like. It is not easy to effect collective coating while correctly keeping the state of alignment of the optical fibers. Methods of improving this coating are disclosed in Japanese Patent Application Laid-Open No. HEI 6-63483 and Japanese Patent Application Laid-Open No. HEI 7-209565, for example.
However, in the gap between the optical fibers and the die, if the difference between the gap in the thickness direction and that in the width direction increases, then a difference occurs between the resin pressure in the thickness direction and that in the width direction, whereby the state of arrangement of the optical fibers may be disordered, or irregularities may occur in the coating surface. It is an object of the present invention to provide a method of making a ribbon type coated optical fiber which can overcome such problems and yield favorable states of surface coating and arrangement.
This method of making a ribbon type coated optical fiber is a method of making a ribbon type coated optical fiber comprising the step of inserting a plurality of optical fibers flatly arranged parallel to each other through a die orifice of a coating apparatus so as to collectively coat the optical fibers with a coating resin, wherein the ratio W/H of a clearance W in a width direction between an inner surface of the die orifice and an outer surface of the optical fibers to a clearance H in a thickness direction therebetween is set within the range of 1.0 to 2.5.
According to this manufacturing method, in the clearance between the die orifice and the flatly arranged optical fibers inserted therethrough, the ratio W/H of the clearance W in the width direction to the clearance H in the thickness direction is defined within the range of 1.0 to 2.5, preferably within the range of 1.2 to 1.9.
As a result of such selection, respective resin pressures in the clearances H and W become substantially even, whereby the state of arrangement of the optical fibers can be held correctly, and the coating surface can be formed smoothly. As the coating resin flows from the higher pressure side to the lower pressure side within the die, the optical fibers shift along the flow of the resin. If a plurality of optical fibers shift, then their arrangement may be disordered, or the amount of coating of the resin may change so that irregularities occur in the surface. With in the range where the clearance ratio W/H is 1.0 to 2.5, such problems can be suppressed.
One of the factors responsible for the difference in resin pressure occurring between the clearances H and W is the difference in the gap (clearance) formed between the optical fibers and the die orifice. In general, if the clearances H and W differ from each other, the resin pressure will be higher on the narrower clearance side and lower on the broader clearance side.
Another factor responsible for the difference in resin pressure occurring between the clearances H and W is the form of arrangement of optical fibers at the time when they are passed through the die orifice. For example, in the case where the optical fibers are inserted therethrough while being arranged flatly, there is empirically a tendency that, even if the respective clearances H and W in the thickness and width directions are equal to each other, the resin pressure in the thickness direction becomes lower, whereas that in the width direction becomes higher. As a consequence, it is important that the clearances H and W be determined in view of these factors, so as to make the resin pressures even.
In the above-mentioned coating method, the difference in resin pressure is also influenced by the melt viscosity and resin pressure at the time of coating of the coating resin, and affects the arrangement of the optical fibers and the smoothness in their surface. It is preferred that the coating resin be supplied to the die orifice while the melt viscosity of the coating resin is held within the range of 1000 to 4000 cps, and the range of 2000 to 3000 cps is optimal. It becomes difficult to stably keep the form of the ribbon if the melt viscosity is 1000 cps or less, whereas a melt viscosity of 4000 cps or greater is inappropriate for high-speed coating. Also, it is preferred that the resin be supplied to the die orifice while the resin pressure is held within the range of 0.5 to 3.0 kg/cm2, and the range of 0.8 to 2.0 kg/cm2 is optimal. A resin pressure of 0.5 kg/cm2 or less is inappropriate for high-speed coating, whereas it becomes difficult to stably keep the form of the ribbon if the resin pressure is 3.0 kg/cm2 or greater.
The method of making a ribbon type coated optical fiber may comprise a step of, while flatly arranging a plurality of optical fibers parallel to each other, inserting the optical fibers through a die orifice of a sub-unit coating apparatus and coating the optical fibers with an inner layer coating resin so as to form at least two sub-units; and a step of, while flatly arranging at least two sub-units parallel to each other, inserting the sub-units through a die orifice of an outer layer coating apparatus and collectively coating the sub-units with an outer layer coating resin so as to form the ribbon type coated optical fiber; wherein the ratio Ws/Hs of a clearance Ws in a width direction between an inner surface of the sub-unit die orifice and an outer surface of the sub-units to a clearance Hs in a thickness direction therebetween is set within the range of 1.0 to 2.5, and wherein the ratio Wg/Hg of a clearance Wg in the width direction between an inner surface of the outer layer die orifice and the outer surface of the sub-units to a clearance Hg in the thickness direction therebetween is set within the range of 1.0 to 2.5.
According to this manufacturing method, the step of coating the outer periphery of a plurality of optical fibers with the inner layer resin to form sub-units and the step of coating the outer periphery of at least two sub-units with the outer layer resin to form the ribbon type coated optical fiber are under the same condition as the first manufacturing method mentioned above.
As a consequence, the resin pressure can be made substantially even in each of the sub-unit die orifice and the outer layer die orifice, whereby the state of arrangement of optical fibers or sub-units can be kept correctly, and the coating surface can be formed smoothly.
Also, in the method mentioned above, it is preferred that, on a disc-shaped die disposed within a cylindrical sleeve, a substantially cylindrical nipple be disposed so as to form a gap between an outer peripheral face of the nipple and an inner peripheral face of the sleeve, and a resin be introduced from the gap toward a die orifice of the die, so that a plurality of the above-mentioned optical fibers positioned within the die orifice are coated with this resin.